L 28 Electricity and Magnetism [6] magnetism Faraday’s Law of Electromagnetic Induction –induced currents –electric generator –eddy currents Electromagnetic Waves (Maxwell & Hertz) 1

Basic facts of Magnetism Oersted discovered that a compass needle responded to the a current in a loop of wire Ampere deduced the law describing how a magnetic field is produced by the current in a wire magnetic field lines are always closed loops – no isolated magnetic poles; magnets always have a north and south pole permanent magnets: the currents are atomic currents – due to electrons spinning in atoms - these currents are always there electromagnets: currents in wires produce magnetic fields 2

Faraday’s Law of Electromagnetic Induction Faraday wondered if the magnetic field due to the current in one coil could regulate the current in an adjacent coil. He was correct, with one important qualification: the magnetic field must be changing in some way to produce a current the phenomenon that a changing magnetic field can produce a current is called electromagnetic induction Michael Faraday ( ) 3

Induced currents (a) When a current is turned on or off in coil A, a magnetic field is produced which also passes through coil B. A current then briefly appears in coil B The current in coil B is called an induced current. The current in B is only present when the current in A is turned on or off, that is, when the current in A is changing A magnetic field lines current indicator switch battery B 4

Induced currents (b) a)No current is induced if the magnet is stationary. b)When the magnet is pushed toward the coil or pulled away from it, an induced current appears in the coil. c)The induced current only appears when the magnet is being moved (a) (b) (c) 5

Induced currents (c) If an AC (time varying) current is used in the primary circuit, a current is induced in the secondary windings. If the current in the primary windings were DC, there would be NO induced current in the secondary circuit. Levitated coil demo 6

electric generators When a coil is rotated in a magnetic field, an induced current appears in it. This is how electricity is generated. Some external source of energy is needed to rotate the turbine which turns the coil. 7

The transformer The voltage on the secondary depends on the number of turns on the primary and secondary. Step-up  the secondary has more turns than the primary Step-down  the secondary has less turns than the primary 8

Eddy currents Eddy currents are induced in conductors if time- varying magnetic fields are present As the magnet falls the magnetic field strength at the plate increases 9 Falling magnet Eddy currents Induced magnetic field Copper plate

Eddy currents  application An induction stove uses eddy currents to cook food Only the metal pot gets hot, not the glass pot or the stove. 10

slotted copper pipe bar magnet Floating magnet – induced currents As the magnet falls, it induces currents in the copper pipe known as eddy currents. These eddy currents produce a magnetic field that opposes the field of the falling magnet, so the magnet does not accelerate but descends slowly. 11

The Laws of Electricity and Magnetism Laws of electricity –electric charges produce electric fields (Coulomb) –electric fields begin and end on charges Laws of magnetism –currents produce magnetic fields (Ampere) –magnetic field lines are closed loops –a changing magnetic field can produce a current (induced currents) (Faraday) –A changing electric field can produce a magnetic field (Maxwell) 12

ELECTROMAGNETIC (EM) WAVES Faraday laid the groundwork with the discovery of electromagnetic induction, Maxwell added the last piece. Heinrich Hertz showed experimentally in 1886 that EM waves do exist. James Clerk Maxwell in 1865 predicted theoretically that EM waves should exist. EMWAVESLIGHT 13

Electromagnetic (EM) waves Mechanical wave: a disturbance that propagates in a medium (eg, water, strings, air) An electromagnetic wave is a combination of electric and magnetic fields that oscillate together in space (no medium) and time in a synchronous manner, and propagate at the speed of light 3 ×10 8 m/s or 186,000 miles/s. EM waves include radio, microwaves, x-rays, light waves, thermal waves, gamma rays 14

the generation of an electromagnetic wave wave emitter e.g. antenna electric field magnetic field The time varying electric field generated the time varying magnetic field which generates the time varying electric field and so on and so on

EM waves: transverse the electromagnetic wave is a transverse wave, the electric and magnetic fields oscillate in the direction perpendicular to the direction of propagation E field B field direction of propagation 16

Electromagnetic waves the EM wave propagates because the electric field recreates the magnetic field (Maxwell) and the magnetic field recreates the electric field (Faraday) The EM wave is self-sustaining an electromagnetic wave has an electric field and a magnetic field component, which are perpendicular to each other and to the direction of propagation. 17

How radio waves are produced Dipole Antenna transmission line High Frequency Oscillator 18 An oscillating voltage applied to the antenna makes the charges in the antenna vibrate up and down sending out a synchronized pattern of electric and magnetic fields.

Electromagnetic Waves Antenna: emits waves EM WAVE: time and space varying electric and magnetic fields moving through space at the speed of light, c = 3 x 10 8 m/s = 186,000 miles/sec 19

Radio antenna the EM wave causes the electrons in the receiving antenna to oscillate at the same frequency the amplifier converts the electrical signal to sound waves Sound waves are transformed to an electrical signal which is amplified and sent to the transmitter 20

The periodic wave relation: c =  f c = 3 ×10 8 m/s is the speed of light Example: –What is the wavelength of an electromagnetic wave having a frequency f = 1 MHz (10 6 Hz)? –Solution: 21 The periodic wave relation applies to electromagnetic waves

Electromagnetic spectrum  = c Visible light 22

AM radio: 535 KHz – 1.7 MHz FM radio: 88 – 108 MHz GPS: 1227 and 1575 MHz Cell phones: 824 MHz – 2 GHz 1 vibration per second = 1 Hertz (Hz) 1 KHz (kilohertz) = 10 3 Hz 1 MHz (megahertz) = 10 6 Hz 1 GHz (gigahertz) = 10 9 Hz Common frequency bands 23

Microwaves are in the frequency range of a few billion Hz or wavelengths of about several cm (about the same range as radar  the “Radarange” How do microwaves heat water? Remember that the water molecule has a positive end and a negative end. The electric field of the microwave grabs onto these charges and shakes them violently a few billion times each second all this shaking energizes the molecules making the water hotter and hotter. 24